In ancient Grece « νανος » (nanos) refers to the world of dwarves. In this miniature world, objects are already complex assemblies of many atoms.
Today, nano is first a unit of length, on billionth (10-9) of a meter. This is a size close to the diameter of a carbon nanotube or the diameter of ADN double helix, which contains the information of the genetic code. It is also the size of a great number of organic molecules.
There are many other examples of nano-objects coming from chemistry or magnetism. Most of these objects have the same size and can be observed with the same tools. Fabrication techniques and assembly methods share also some similarities.
This is why scientific discipline converge at the nanometer-scale. On this scale, some of the concepts and observation techniques are shared by all natural sciences. nanoscience is in essence a pluridisciplinary field.